I never got any sort of official source on this, but I noticed from the 3G to the 3GS it seemed like all they did was rework the volume so that 50% external speaker volume on the 3GS was equivalent to 100% on the 3G. Sounds which were previously not distorted with the 3G suddenly became distorted, etc. They redesigned the speaker of course when they went to the iPhone 4 and got a much cleaner sound.
But I have a feeling they did the cheap trick again from the 4 to the 4S—raising volume in software without actually improving the speaker. It kinda hurts my ears and I have to turn it down now.
I could never use speaker on my iPhone 4 because I could not hear comfortably ... but just used speaker on the 4S and it was crystal clear and PLENTY loud enough. IMHO, the clarity improvement is HUGE as is the volume. (btw I had to use speaker because I refused to take the protective plastic off until I got protection LOL Yeah I am hopeless)